In Frequency Division Multiplexing (FDM) communication systems, the available spectral bandwidth W is divided into a number of spaced sub-carriers, f1, . . . , fN, which are used to transmit information. Specifically, information bits are first mapped to complex FDM symbols B1, . . . , BN. The signal to be transmitted, S(t), is constructed by individually modulating those symbols onto the sub-carriers over an FDM symbol duration, that is,S(t)=Σk=1N|Bk|cos [2πfkt+θk],where |Bk| and θk are the amplitude and the phase of complex symbol Bk, respectively, and t is the time variable. Orthogonal Frequency Division Multiplexing (OFDM) is one particular example of FDM.
FIG. 1 illustrates a known system 100 for generating and transmitting an OFDM signal S(t). In the known system 100, a digital signal generator 112, generates a sequence of baseband discrete complex samples of S(t), which are then converted to an analog continuous signal through use of a digital-to-analog converter 114. The analog signal generated by the D/A converter 114 is passed through a low-pass filter (LPF) 115, mixed to the carrier frequency by mixer 116, amplified with a power amplifier 118, and finally transmitted over the communication channel 120. The LPF 115 is normally selected as a function of the frequency of the signal generated by the digital signal generator 112.
In the known system, information to be transmitted on sub-carriers is combined in the digital domain so that by the time digital to analog conversion occurs distinct sub-carrier symbols do not exist, e.g., separate symbols corresponding to different sub-carriers are not available to be subject to separate and distinct digital to analog conversion operations and/or separate analog signal processing operations.
One major drawback of the known OFDM signal generation technique is the high peak-to-average ratio of the transmitted signal to be amplified. Loosely speaking, the peak-to-average ratio is the ratio of the maximum and the average powers of a signal. In general, the signal reception capability depends on the average power of the signal. However, to avoid nonlinear distortion such as signal clipping, the power amplifier at the transmitter normally has to operate linearly across the full dynamic signal range of the generated signal. This usually requires use of a class A power amplifier. As a result of the linear nature of the power amplifier 118, the power consumption of the power amplifier mainly depends on the maximum transmission power. Hence, the peak-to-average ratio is an important measure of power consumption given the quality requirement of signal reception.
In the OFDM system 100, the analog signal to be amplified is the sum of many sinusoid waveforms, e.g., sub-carrier signal. Assuming complex OFDM symbols B1, . . . , BN are independent random variables, the analog signal at a given time instant will tend to be a Gaussian distributed random variable, which is well recognized to have a large peak-to-average ratio. Hence, the transmission of the OFDM signals generally consumes a significant amount of power, which is very undesirable, e.g., for mobile transmitters using battery as power supply. Various methods have been proposed to reduce the peak-to-average ratio of the OFDM signals. The basic ideas in these methods is to arrange complex symbols B1, . . . , BN appropriately to minimize the peak to average ratio. However, in such methods, the fundamental structure of signal transmission of combining sub-carrier signals first and then power amplifying the combined signal is normally the same as shown in FIG. 1.
In order to overcome some of the power amplification problems of the FIG. 1 system, a system such as the one shown in FIG. 2 was developed. FIG. 2 illustrates a frequency division multiplexer signal generation and transmission system capable of generating and transmitting OFDM signals. As illustrated in FIG. 2, information bits to be transmitted on various sub-carriers are first mapped to complex OFDM symbols B1, . . . , BN, e.g., one symbol per sub-carrier for each symbol period, by a digital symbol generator (DSG) 202. Each OFDM symbol Bk (where 1<k<N) is then modulated to a corresponding sub-carrier fk using a corresponding sinusoidal signal generator 214, 214′ of signal generator module 204, thereby generating an analog sinusoid signal for one symbol duration for each sub-carrier. The symbol duration is equal to the inverse of the spacing between two adjacent sub-carriers, plus the duration of a cyclic prefix portion when present. Each complex OFDM symbol to be transmitted is used to convey information bits to be communicated.
In the FIG. 2 system, the sinusoid signal generators for each sub-carrier are fixed frequency signal generators. The signals (SS1-SSN) of the sub-carriers are power amplified individually. The amplification of individual sub-carrier signals is performed in parallel, e.g., by using different sub-carrier signal paths, each sub-carrier signal path including a single power amplification module 206, 206′ and a corresponding fixed filter 218, 218′. Each of the fixed filters 218, 218′ correspond to the particular subcarrier frequency of the subcarrier path and is used to reject high order harmonics relative to the frequency of the subcarrier to which the filter 218, 218′ corresponds. In cases where the filters 218, 218′ are implemented as bandpass filters, they will normally have a passband centered around the corresponding subcarrier frequency and a bandwidth corresponding to the distance between subcarrier frequencies. In such a case, if the subcarrier frequency spacing is Δf the filter 218 will normally be a fixed filter with a center frequency centered around f1 and a bandwidth of approximately Δf. Similarly, in such a case filter 218′ corresponding to subcarrier N, will normally be a fixed filter with a center frequency centered around fN and a bandwidth of approximately Δf. Fixed filters are relatively inexpensive to implement while matching filter cutoff regions to particular subcarrier frequencies has the advantage of reducing noise and potential interference between subcarrier signals which are later combined exclude signals, e.g., high order harmonics or other signals.
The use of fixed filters of the type described in regard to the FIG. 2 system works well when subcarrier signal paths correspond to a single fixed frequency.
Unfortunately, it is often the case that the frequencies on which a particular device may want to transmit information can change with time. In the case of a mobile device such as a PDA or other mobile communications device, the subcarrier frequencies upon which the mobile device is to transmit at any given time may change due, e.g., to changes in channel transmission allocations and/or the use of frequency hopping schemes.
In the case of a base station where the same set of N subcarrier frequencies is used on a continuous basis to transmit data, e.g., to a plurality of mobile devices, it may be practical to use N dedicated fixed subcarrier amplification and filtering signal path as shown in FIG. 2. This is because all or most of the N subcarriers will be used at any given time, e.g., with the data intended for different mobile devices being directed to the particular subcarrier signal path that corresponds to the frequencies allocated to the particular intended mobile device at any point in time.
Unlike base stations, mobile devices often use, at any given time, a small subset, e.g., M, of the total N subcarrier frequencies used in a cell at any given time where N>M. From cost, size and other reasons such as weight, in various devices, but particularly mobile devices, it is often impractical to provide a separate dedicated transmitter subcarrier signal path, e.g., amplifier and filter, for each of N possible subcarrier signals. This is particularly the case when only a small subset, e.g., M, of potential subcarrier frequencies N, may be used for transmission purposes at any given time.
In view of the above discussion, there is a need for improved frequency division multiplexed signal generation and transmission techniques. While the techniques should provide for low peak-to-average power ratios and therefore improved energy efficiency during power amplification stages of signal generation they should also be practical in terms of hardware implementation and not require separate subcarrier signal paths for each potential subcarrier frequency which may be used. It is desirable that at least some of the new methods and apparatus be suitable for use with frequency hopping schemes and OFDM signals and that at least some methods be well suited for use in implementing mobile communications devices, e.g., at reasonable cost.